Forbid dynamic arrays in boolean evaluation contexts

[bearophile]

deadalnix:

>> On the other hand if you compile a program that uses null 
>> instead of [] you see some differences. In the current dmd 
>> compiler returning null is more efficient. I have seen code 
>> where this difference in performance matters:
> ...
> That is a compiler bug isn't it ?

I don't know. Maybe it's just a missed optimization :-)

Bye,
bearophile